This invention relates to a pneumatic radial tire. More particularly it relates to a pneumatic radial tire including laminated belts of at least four rubber coated cord layers arranged for reinforcing a tread about a crown region of a radial carcass to be able to effectively prevent separations liable to occur at belt ends so as to improve the durability, of the, tire without increasing diameter enlargement. The invention is advantageously applicable to heavy duty vehicles such as trucks and buses and ultraheavy duty vehicles such as construction and earth moving vehicles.
In general, a pneumatic radial tire includes a plurality of cord layers as reinforcing belts whose rubber coated cords are arranged in parallel with each other. The cord layers are arranged about a crown region of a radial carcass to be laminated so that the rubber coated cords intersect with each other over a crown circumference to reinforce the tread of the tire. With such a pneumatic radial tire, however, separation faults often occur in the proximity of width ends of the belts.
In order to overcome this problem, there have been many proposals with respect to cord inclined angles of belt layers, forms of belts, shapes and arrangements of cord layers and the like. Although satisfactory results have been obtained with relatively small tires, for example, pneumatic radial tires for passenger cars, separations apt to occur in the proximity of width ends of belts could not be sufficiently prevented with large tires. It probably results from the fact that a number of rubber coated cord layers are generally used for belts of large tires for trucks and buses or ultralarge tires for earth moving or construction vehicles.
The reinforcing of the tread of a pneumatic radial tire with the aid of belts serves to restrain the diameter enlargement caused by filling with inner pressure into the tire. If the restraint is insufficient, tensile strains occurring in a tread of the tire detrimentally affect wear-resistance and cut-resistance of the tread.
In order restrain the diameter enlargement, it is of course necessary to make small intersecting angles of cords of belts over a crown circumference of a carcass and simultaneously use large modulus cords.
On the other hand, tensile forces acting upon belts made of laminated cord layers having small cord angles cause the belts to deform so as to make smaller the intersecting angles of cords in the so-called "pantograph displacement". However, as side edges of the belts terminate in free ends of the cords, the side edges of the belts are displaced to a greater extent in comparison with centers of the belts. These displacements are opposite to each other according to directions of inclinations of the cords of the laminated cord layers.
Therefore, considerable shearing strains occur in the rubber embraced between adjacent cord layers with different cord inclined angles in the proximity of side ends of the belts under tensile forces of the belts. The shearing strains cause the separations at ends of the belts.
It has been therefore difficult to simultaneously accomplish the restraint of the diameter enlargement and the prevention of separation at belt ends.